在 Golang 中重新切片切片 [英] Re-slicing slices in Golang

问题描述

最近学了 Go 语言，现在对下面的代码很困惑:

I recently picked up the Go language, and now I am confused with the following code:

package main

import "fmt"

func main() {
    a := make([]int, 5)
    printSlice("a", a)
    b := make([]int, 0, 5)
    printSlice("b", b)
    c := b[:2]
    printSlice("c", c)
    d := c[2:5]
    printSlice("d", d)
}

func printSlice(s string, x []int) {
    fmt.Printf("%s len=%d cap=%d %v\n",
        s, len(x), cap(x), x)
}

结果:

a len=5 cap=5 [0 0 0 0 0]
b len=0 cap=5 []
c len=2 cap=5 [0 0] //why the capacity of c not 2 but 5 instead
d len=3 cap=3 [0 0 0]

推荐答案

c 是取自数组 b 的切片.这不是副本，而是 b 的第 2 个元素上方的窗口.

c is a slice taken from the array b. This isn't a copy, but just a window over the 2 first elements of b.

由于 b 的容量为 5，c 可以扩展到其他 3 个位置(实际上它创建了一个新切片，但在内存中的相同位置).

As b has a capacity of 5, c could be extended to take the 3 other places (in fact it makes a new slice but over the same place in memory).

切片的最大容量是底层数组的容量减去切片在数组中的起始位置:

The maximal capacity of the slice is the capacity of the underlying array minus the position of the start of the slice in the array :

 array : [0 0 0 0 0 0 0 0 0 0 0 0]
 array :  <----   capacity   --->
 slice :     [0 0 0 0]
 slice :      <---- capacity ---> 

也许这个程序会更清楚地表明 c 和 d 只是 b 上的窗口:

Maybe this program will make it more clear that c and d are just windows over b :

func main() {
    b := make([]int, 0, 5)
    c := b[:2]
    d := c[1:5] // this is equivalent to d := b[1:5]
    d[0] = 1
    printSlice("c", c)
    printSlice("d", d)
}

输出:

c len=2 cap=5 [0 1] // modifying d has modified c
d len=4 cap=4 [1 0 0 0] 

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